The present invention relates to a coaxial rotor system, and more particularly to a coaxial rotor system with closely spaced articulated rotors.
Future military forces require enhanced vertical lift capabilities in a compact package. The CH-53E is currently the world's largest shipboard compatible helicopter. A significant consideration in the design of the CH-53E was shipboard compatibility. The CH-53E effectively defines the maximum aircraft spatial capacity which will fit on the elevators and in the hangar deck of United States Marine Corps Amphibious Assault Ships, more commonly called an LHA or LHD. Emerging payload weight requirements are beyond the growth capabilities of the CH-53E while maintaining current shipboard compatibility requirements. Thus, a conventional helicopter configuration like the CH-53E would be too large to fit in the hangar deck or on the elevator of an LHA or LHD.
Conventional coaxial rotor systems are exceeding efficient as lift generating mechanism for a heavy lift VTOL aircraft. There are no power losses to an anti torque device and rotor efficiency is somewhat improved relative to a single rotor due to swirl recovery. The aircraft also has a much lower foot print due to the lack of a tail rotor and supporting boom structure. Disadvantageously, conventional dual counter rotating coaxial rotor systems require a relatively large separation between the rotor systems. This drives the height of a coaxial rotor aircraft to be taller than that of a single rotor aircraft.
Typically the rotor or disks of a conventional dual counter rotating coaxial rotor system are axially spaced a distance of approximately 10 percent of the rotor diameter. Such a separation is required to provide adequate space for differential rotor blade flapping and bending to assure clearance therebetween regardless of aircraft maneuver. The blade tip position of a conventional coaxial rotor system is determined by the natural equilibrium of aerodynamic and inertial forces acting on the blade. Since the rotors are counter rotating, many maneuvers cause mirror image, or differential, rotor tilt, which reduces tips separation at some point in the rotation. It has been found from decades of industry experience that a hub separation distance of 10% of rotor diameter is adequate for most transport types of aircraft. Disadvantageously, application of such rotor spacing to a heavy lift VTOL aircraft which are capable of emerging vertical lift requirements result in an aircraft which will likely not meet current shipboard height compatibility restrictions.
Accordingly, it is desirable to provide an affordable heavy lift VTOL aircraft with low to moderate risk technologies while being compatible with current shipboard restrictions.